But the best thing is to see what your "head" has printed on his/her door sign and tell them it's the other one.

Anyway, if your head-teacher is anything like mine, the teacher suffix is a laugh since he doesn't teach, except when cover gets desperate and then displays a complete inability to teach; moreover he shows little understanding of teaching. Perhaps the correct title would be to remove the "teacher" suffix and add the "dick" prefix.

Chambers, the most authoritative dictionary of contemporary British English usage, has "head teacher", but makes no mention of "headteacher". The latter is a coinage which owes its existence to the pseudo-leftie bourgeois media, which refuse to use "headmaster" and "headmistress" because they are marked for sex, or, in pseudo-leftie bourgeois media jargon, are not "gender neutral", and not being "gender neutral" is a mortal sin. Schools may have a head boy and a head girl, too, but never a "headboy" or a "headgirl".